10 crayons cost $8.50. Which equation would help determine the cost of 15 crayons?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 10 crayons. We want to know the cost of 15 crayons. We can write the numbers of crayons as a proportion: $\dfrac{10}{15}$ We know 10 crayons costs $8.50. We can let $x$ represent the unknown cost of 15 crayons. The proportion of these costs can be expressed as: $\dfrac{\$8.50}{x}$ The cost changes along with the number of crayons purchased, and so the two proportions are equivalent. $\dfrac{10}{15} = \dfrac{\$8.50}{x}$